SOHC.co.uk Forums > CB750
CB750 K1 Valve Timing Question
cbxman:
Not being an expert at all. See my bumbling earlier in this thread.
The theory may be fine if you are familiar with fluid dynamics and can understand it, but simple test surely is to check plug condition against an airscrew setting. that should settle it....
Jerry
K2-K6:
Yes and easy to check. Turning in until it stops the cylinder just goes far too rich and black smokes like it's on choke (unburnt HC) and the opposite as you back the screw out.
The others on thread do offer a more scientific basis with anecdotal supporting analysis though. All academic though as thread age is older in that case.
Jets are more akin to siphon though rather than anything more sophisticated. Try sucking liquid up with a straw, then put a hole in the side of the straw, you'll just get air bleeding in if air enters above liquid level.
I don't understand the reasoning behind changing emulsion tube holes in response to ethanol content. They are just there for atomisation of fuel droplets in the mixture (left as they are would seem reasonable) and doesn't doesn't deal with combustion characteristics that seem associated with ethanol mix.
Essentially "emulsion" arrangement helps produce a stratified charge to burn effectively. Meaning, each fuel droplet would be surrounded by 14.7 parts oxygen content air, the more evenly and homogeneous this is the more consistent the burn rate across the combustion chamber. It wouldn't appear to need any shift there to accommodate it, and certainly would need some very concise observations to understand the effects.
cbxman:
Nigel,
It seems that the air entering the throat by way of the twist grip is the air that sets the 14.7, demand from the needle jet. I guess the emulsion tube makes the petrol flow out of the needle jet in smoother and, as you say, homogeneous way. However enlarging the emulsion tube holes even ever so slightly must add air to the mix, which I guess means leaning the overall mixture. Mark suggests that ethanol in the fuel makes the combustion burn cooler. So maybe the extra leanness is compensating for the coolness and makes the combustion run hotter, maybe ????
Perhaps a reduction in main jet size is too crude for such subtlety?
Jerry
K2-K6:
Obviously it's a complex topic, interesting piece for discussion
"Ethanol molecules include oxygen atoms, whereas gasoline molecules don't. That's part of the reason why ethanol has less energy than gasoline. Another effect of the oxygen from ethanol is that ethanol blends tend to run "leaner" than pure gasoline because there is more oxygen available in the fuel-air mixture. If your engine is not able to compensate by reducing the incoming airflow, the resulting combustion conditions in the engine cylinder may be less than ideal. Newer vehicles are generally designed to take care of this automatically, but older engines may need a bit of manual adjustment to get the air-fuel mixture just right.
Some people have reported engines overheating when ethanol blends are used, suggesting that ethanol burns "hotter." This is a bit mysterious since ethanol contains less energy per unit volume than gasoline, and the flame temperature of ethanol is more than 40°C cooler than gasoline. The most likely explanation is related to the air/fuel ratio. Most engines are designed to run with an excess of fuel relative to the amount of air (a "rich" mixture); experience has shown that this leads to higher power output and cooler engine temperatures. When ethanol blends are used, newer engines are equipped with sensors to adjust the air/fuel ratio automatically. Older vehicles and small engines may not be equipped to do this, resulting in a "leaner" burn that may increase engine temperatures and/or reduce engine power. A simple adjustment to the fuel system to "richen" the mixture can often fix this problem."
This effectively contradicts adding more air.
cbxman:
Nigel,
Interesting stuff, so why open the emulsifier holes. That would make sense of Mark's explanation of adding extra air with the airscrew which enriches the idle mixture. However, we know that adding extra air via the airscrew to the idle jet actually leans it out.
Maybe the difference between how an idle jet works and how a main/needle jet works is the answer.
Somewhere, the emulsion tube function is explained, maybe in that same thread. The lower holes are under the fuel height in the bowl, the upper (smaller ones to be opened up) are above the fuel height. This would imply weakening the mixture if we are observe the equivalent in the idle circuit. Marks explanation of making the fuel bubble up faster would make sense in making it richer.....
Confused
Jerry
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version